Known track assemblies for endless track laying machines include a plurality of links with laterally overlapping ends joined by bushings, pins, oil seals that are interposed between the ends of the bushings and the links, and track shoes which are fixed with bolts and nuts on the links.
The main causes of damage to the bushings are wear and fatigue cracks. In an effort to increase bushing life, the outer peripheral surface and inner circumferential surfaces are hardened to resist wear. The outer peripheral surface of the bushing requires a high hardness and a hardened depth because it is the surface that engages the sprocket for driving the track. The center of the inner circumferential surface requires high fatigue resistance because of the stresses created by the load transfer path from the sprocket into the bushing. The end portions of the inner circumferential surface require high surface hardness because of high stress caused by contact between the bushing and the pin.
In order to meet such severe conditions, bushings for tracks are given various heat treatments.
An example of a method of heat treating a bushing is disclosed in U.S. Pat. No. 5,032,192 issued Jul. 6, 1991 to Tsuchiya et al. The above patent uses a medium carbon steel for the bushing material. The steps for the heat treating process is first comprised of carburizing the bushing and then cooling to ambient temperature. After which, the outer peripheral surface of the bushing is induction-heated so that an entire wall cross-section is heated to a quenching temperature. The entire bushing is then cooled by a quenching liquid so that the bushing is evenly hardened. After hardening, the bushing is tempered.
Another example of a method of heat treating a bushing is disclosed in U.S. Pat. No. 5,049,207 issued Sep. 17, 1991 to Sahara et al. This example also uses a medium carbon steel for the bushing. In this process, the bushing is first induction hardened from the outer peripheral surface until the inner circumferential surface reaches the austenitic transformation temperature of 1540 degrees Fahrenheit. Then, the bushing is cooled by quenching across the entire thickness of the bushing. The bushing is then tempered in a furnace between a specified temperature range so as not to impair the effective hardness acquired by the heat treating process. The last step of this process is to induction temper the bushing from the inner circumferential surface using the conductive heat to temper the outer peripheral surface at a temperature lower than that on the inner circumferential surface.
The methods described above require multiple heating steps, moving the bushing from one heating apparatus to another, and a considerable consumption of manufacturing time. The result being increased labor and manufacturing cost, and a waste of energy and resources.
The present invention is directed to overcoming one or more of the problems as set forth above.